1. Field of the Invention
The present invention relates to an image reading apparatus and, more particularly, to an image reading apparatus for converting an original density per pixel obtained by reading an original image into a digital value and outputting the digital value to an external device.
2. Related Background Art
In a conventional image reading apparatus, an original is illuminated, and illuminated image information is photoelectrically converted into a digital value (e.g., n bits/pixel), and the digital value is modulated into one of the following values:
(1) multi-value information (to be referred to as multi-values hereinafter) representing one pixel by n bits;
(2) binary information (to be referred to as a binary value hereinafter) representing one pixel as "1" or "0" information by using a predetermined digital value as a slice level; and
(3) binary dither information (to be referred to as dither hereinafter) representing one pixel by "1" or "0" information by using each digital value of an m .times. n matrix as a slice level.
Another conventional image reading apparatus is known in which a data format of an image read by this apparatus is designated by an external device. For example, a binary mode, a dither mode, and a multi-value mode can be designated from the external device.
A bit width in the multi-value mode is defined by that of an internal A/D converter in the image reading apparatus independently of the bit width required for the external device. If 6-bit multi-value data is requested for an image reading apparatus having, e.g., an 8-bit A/D converter, this request cannot be accepted. In this case, the external device receives 8-bit data and then converts the 8-bit data into 6-bit data.
In the image reading apparatus having, e.g., an 8-bit A/D converter, information (8 bits/pixel) is transferred to the external device in the multi-value mode, and information (8 bits/8 pixels) is transferred to the external device in the binary and dither modes.
The conventional image reading apparatus reads an original and performs image modulation on the basis of a predetermined transfer format (8 bits/pixel in the multi-value mode; and 8 bits/8 pixels in the binary and dither modes).
The conventional data transfer scheme has three types of data, i.e., multi-values, a binary value, and dither, and three transfer formats, i.e., 8 bits/pixel in the multivalue mode, 8 bits/8 pixels in the binary mode, and 8 bits/8 pixels in the dither mode. Therefore, a circuit for converting an image to be transmitted to the external device into 8 bits/pixel and 8 bits/8 pixels, and a circuit for selecting one of the transfer formats are required.
In particular, in a reading apparatus for independently reading a character portion and a photographic portion of an original, a circuit for discriminating the character portion (binary information) from the photographic portion (dither information) is required. As a result, hardware of the reading apparatus becomes bulky, and a means for designating a transfer format and a data format (i.e., a control code supplied from the external device to the reading apparatus when the external device accesses the reading apparatus; and a control code for acknowledging a transfer format to the external device when the reading apparatus includes a means for designating image reading) is required.